I'm trying to understand the "legal fiction" wherein we declare any chometz we have as ownerless like the dust of the earth. Is this halachically acceptable? How can a person declare something in his possession, possibly locked up in his house to be ownerless?
First if it is a "legal fiction" then you are in fact transgressing biblical and Rabbinical commandments regarding possessing Hametz over the course of Pesach.
First you are referring to the Bitul Hametz that we all recite. However it is important to note that the formula of that specifically says,
It therefore does not cover any Hametz that you have seen, know full well where it is, and have no intention of destroying.
Likewise you seem to believe that the sale of Hametz is also a legal fiction. In truth for many years it was and many many major halakhic authorities spoke out against the practice. Even today there are many people who do not rely upon the sale of Hametz. However, for those that do, they have what to rely upon so long as the sale is not a legal fiction, either to them or to the one buying the Hametz. Here is an article that goes in depth in to the various halakhot that are involved in sales of hametz by any reliable agent of sale today. By Rabbi Peretz Moncharsh who's bio can be found here.
When making something ownerless it is just that ownerless. In Halacha, to take possession of something you must pick it up or otherwise possess it. If you were to move something that has chametz, you would be regaining your ownership of it. If it was sold, you wouldn't be since it has a new owner already.